1. Field of the Invention
The present invention relates to an electrode structure for an n-type Al.sub.x Ga.sub.y In.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor device, one of the Group III-V compound semiconductor devices containing nitrides, and a method for fabricating the same. More specifically, the present invention relates to an electrode structure having a substantially ideal ohmic contact showing an extremely small contact resistance between a semiconductor layer and an electrode layer and exhibiting excellent thermal stability and a method for fabricating the same.
2. Description of the Related Art
Generally, in fabricating an electrode structure for an Al.sub.x Ga.sub.y In.sub.1-x-y N (0.ltoreq.x1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor device, nitrogen, one of the elements constituting the semiconductor, is likely to dissociate from the surface of a semiconductor layer in the electrode structure when the semiconductor layer is formed. Therefore, it is difficult to produce crystals satisfying a desirable stoichiometric ratio. When the dissociation of nitrogen generates the vacancies inside the crystal structure of the semiconductor layer, the conductivity type of the semiconductor layer turns into n-type. The resulting n-type semiconductor layer incorporates defective crystals. Thus, if a semiconductor device is fabricated by forming an electrode layer on such a semiconductor layer, the semiconductor device cannot exhibit satisfactory heat stability.
In recent years, the technologies for the crystal growth have been developed so as to considerably improve various characteristics of the resulting crystal structure. As a result, the number of the vacancies caused by the dissociation of the nitrogen atoms has been substantially reduced. In this case, the carrier density can be controlled by doping the semiconductor layer with an n-type impurity such as silicon (Si) and germanium (Ge), thereby obtaining crystals having a carrier density of about 10.sup.19 cm.sup.-3. However, the carrier density at such a level is insufficient to form an ohmic contact between an electrode layer and the semiconductor layer in forming the electrode structure. Therefore, the development of a semiconductor layer having an even higher carrier density is eagerly demanded.
On the other hand, various electrode structures usable for Group III-V compound semiconductor devices containing nitrides such as blue-light-emitting diodes have been conventionally developed. In the proposed electrode structures, various kinds of metals are used to form the electrode layer. For example, in order to form the electrode layer for an n-type Al.sub.x Ga.sub.y In.sub.1-x-y N (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, x+y.ltoreq.1) semiconductor device, aluminum (Al) is most commonly employed ("P-type Conduction in Mg-Doped GaN Treated with Low-Energy Electron Beam Irradiation", H. Amano et al., Jpn. J. Appl. Phys. 28 p. L2112 (1989)). Japanese Laid-Open Patent Publication No. 5-291621 discloses that chrome (Cr), titanium (Ti) and indium (In) are also usable as the metals for forming the electrode layer in place of A1. However, any of these metals for the electrode layer cannot solve the above-mentioned problems. The reason is as follows. Since a satisfactory carrier density cannot be obtained for the semiconductor layer even by the use of these metals for the electrode layer, a contact resistance between the semiconductor layer and the electrode layer increases. In addition, in the electrode structure using these metals for the electrode layer, crystals having an inappropriate stoichiometric ratio are formed in the interface between the semiconductor layer and the electrode layer in some cases. As a result, the electrode structure using these metals for the electrode layer sometimes exhibits unsatisfactory heat stability and high-temperature resistivity. Consequently, these metals for the electrode layer cannot be suitably used as the materials for the electrode layer in the electrode structure for the semiconductor device.
In consideration of these conventional problems, an electrode structure for an n-type semiconductor device having an ideal ohmic contact showing an extremely small contact resistance between a semiconductor layer and an electrode layer and exhibiting excellent thermal stability is eagerly demanded.